Validating User Input in .NET

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Validating User Input
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Overview
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This chapter covers the validation controls used in ASP.NET. These controls make page validation much easier and reduce the amount of code that the developer must write to perform page validation. The ASP.NET team reviewed numerous Web sites to determine the most common types of validation that were taking place on the Web. Most developers were reinventing the wheel to perform validation, so the ASP.NET team decided that Web developers needed a set of validation controls to add to their toolbox. From the start, these controls were designed to detect the version of the browser when used in client-side validation and then render the correct version of HTML for that client browser. These research efforts lead to the development of the six controls covered in this chapter. The examples in this chapter will take a look at each control and explain the most commonly used properties for each control. However, keep in mind that all of the controls share basic properties, such as font, fore color, back color, and so on, so this chapter won't discuss those properties in detail. After you have read this chapter, you will have a firm understanding of validation controls and will be ready to use them in your own Web applications.
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Understanding Validation Controls
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Everything in the .NET Framework is a class, and the validation controls are no exception. All validation controls in the .NET Framework are derived from the BaseValidator class. This class serves as the base abstract class for the validation controls and provides the core implementation for all validation controls that derive from this class. Validation controls always perform validation checking on the server. Validation controls also have complete client-side implementation that allows browsers that support DHTML to perform validation on the client. Client-side validation enhances the validation scheme by checking user input as the user enters data. This allows errors
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to be detected on the client before the form is submitted, preventing the round trip necessary for server-side validation. In addition, more than one validator may be used on a page to validate different aspects. Before you look at the validation controls, step back in time before .NET and take a look at a simplified example of performing validation with ASP. Launch your browser and run the BeforeDotNet.asp code. A screen similar to Figure 6-1 should be displayed.
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Figure 6-1: BeforeDotNet.ASP The following ASP code generated Figure 6-1, which is one example of how validation might be performed on the server using "classic" ASP:<%@ Language=VBScript %> <html> <head> <title>When dinosaurs roamed...</title> <% '-- perform server side validation If Request.Form("Submit") <> Empty Then If Len(Request.Form("First")) = 0 Then Response.Write("Please enter your first name.") Else Response.Write("Hello, " & CStr(Request.Form("First"))) End If End If %> </head> <body> <h3><b>Hungry Minds<b></h3><hr><br> <form method="post" action="BeforeDotNet.asp" name="OldStyle"> First<br> <input name="First" maxlength="15"> <br><hr> <input name="Submit" type="submit"> <br><br> </form>
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</body> </html> This style of validation has a few drawbacks. First, if you have several more controls on this page to validate, the validation routine will become quite lengthy and prone to errors. Second, if you delete or rename your controls on the page, you need to update your validation routine. Also, you may want to perform several different types of validation on the same control's value. As you can see, this adds to your validation routine and requires a fair amount of code to implement. Finally, you have your validation code mixed with the presentation code. As you will soon see, .NET's validation controls overcome all of these limitations. Note You could just as easily have used client-side validation with JScript to save a round trip to the server. However, it is always good practice to validate the values again on the server to prevent any unwanted values from being passed to your server.
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